Video amplifier



Sept. 4, 1945. K `s'czl-uJasmlGER 2,384,253

VIDEO AMPLIFIER I l Filed May 7, '19454 4 Sheets-Sheet 1 Ifgzz. w15.-

OUTPUT OUTPUT ATTORNEY INVENToR lfwvfdldas/wer B" MKM Sept- 4 194.5- K scHLEslNGER 2,384,263

VIDEO AMPLIFIER Fil'ed May 7, 1943 4 Sheets-Sheet 2 OUTPUT NHL '5; PUT.

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OUTPUT SIG/ML INPI/7' IN VEN TOR.

, Afef/GMW Sept. 4, 1945. K scHLEslNGER 2,384,253

VIDEO AMPLIFIER FiledvMay 7, 1943 4 Sheets-Sheet 5 AAA. n

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K. SCHLESINGER VIDEO AMPLIFIER Filed May 7, 1943 4 Sheets-Sheet 4 Kil@ Patented Sept. 4, 1945 UNITED STATES PATENT former. y

VIDEOAMPLIFIEB Kurt Schlamm, 'wat Lafayette, ma., saigner to Radio Corporation of America, a corporation of Delaware Application May' 1, ma, snel-m No. 485,981 l 14' claims. (ci. 11s-m) This invention is directed to electronic ampliers and particularly to an amplifier for handling' a wide band oi' frequencies, such. as finds extensive use in television video circuits. In its particular form, the amplifier includes a plurality of voltage amplifier stages with electronic tubes serving as the coupling elements between successive stages. These coupling tubes are connected between successive stages as cathode followers and they result in splitting the` load capacitances across the voltage amplifier into two parts in such a way as to leave only the plate-to-ground capacitance'of the driver tube as ballast, and, at the same time, to eliminate A the grid-to-ground capacitance of the next amplier by causing it vto reappear across the cathode follower output where it forms a negligible load. l

The amplifier herein to be described is one which permits a substantial increase in gain over circuits heretofore known and used. In addition, the amplier is so constituted as to make possible a. reasonablysubstantial increase in the band width or response frequency range accommodated. In its preferred form, suitable provision is made for high frequency peaking with readily obtainable values of variable condensers which are preferably connected at one side to ground. In such an arrangement these condensers are used in combination with appropriately designed inductance elements. Aspthe amplifier is preferably constituted, series peaking at the output of the coupling tubes is relied upon, for the most part, but, under some circumstances, shunt peaking at the coupling tube inputs is also found feasible and utilized. y

Still further, the ampliiiermakes provisionl for low frequency correction as well as for providing,

where necessary, D. C. coupling between stages. In its preferred form, the amplifier fis so arranged that adequate gain control may be provided without causing any appreciable change in the over-all frequency response and without introducing any noise.

In its broadest form, the amplifier comprises a plurality of thermionic amplifiers of which the output from kone is coupled to the input of anf frequently desirable to provide some degeneration in the circuit and, at the same time, to introduce certain, cathode-to-ground capacity which isl preferably made equal to the plate-to-ground capacity insofar as the time constant of the resistance and capacity elements are concerned.

With such an arrangement. a band stretching effect is conveniently realized.

In further refinement of the circuit, a suitable v volume control arrangement is provided in connection'with the cathode follower stage in' suchy a manner that the picture quality which results from the transmission of the video vsignals through the ampliner is not impaired by the con trol, since the source is so connected as to act' an a r as a low impedance.

Iii :psetll further refinement o f the circuit, there is a switching unit from which a resultant output is derived which may be either positive or negative in sign. In a still further renement, suitable provisionlias been made for operating a voltage regulator in thei'orm of a stabilizing triode in combination with circuits of the foresoins types.

tion to provide an amplier unit which will respond substantially uniformly over a wide range of frequencies and, at the same time, it becomes .an object of the invention to provide an amplier unit wherein provision for peamng at high frequencies is readily made available, while making it also possible to compensate for low frequency distortion.

Still other objects of the invention are those of providing an efficientlyv operating cathode follower amplier with conveniently arranged gain I control instrumentalitieswhich are capable of other, for instance, by way of a triode element having its grid directly connected to the output of the driver stage and with a direct connection to the grid -or' control electrode of the succeedutilization of a desired frequency band, where a number of amplifier stages are utiiued, it is 55y ing stage by way of a cathode follower connection. In order to provide for a more efficient Other objects and advantages of the system will become apparent to those skilled in the art to` which the invention is directed when the folv It accordingly becomes an object of this invenj llowing specifications and claims are considered together with the accompanying drawings. wherein:

Fig. la discloses a resistance-capacity coupled amplifier;

Fig. 1b discloses a modication of Fig. 1a showing a cathode follower coupling between stages;

Fig. 2 is a circuit showing a modification of Fig. lb and provides high frequency correction by cathode degeneration;

Fig. 3 is a series of curves to explain Fig. 2 and to show by curves A, B and C three possible conditions of operation;

Fig. 4 embodies a circuit illustrating high frequency peaking by series and parallel resonance in combination with the cathode follower;

Fig. 5 is a circuit showing one form of low frequency correction;

Fig. 6 discloses a. modification of the circuit of Fi;

Fig. 7 sets forth a circuit modification showing particularly a noiseless gain control system;

Fig. 8 is a modincation of Fig. 7, showing combined use with picture inverter; and,

Fig. 9 sets forth a circuit showing a complete amplier set-up incorporating features of above circuits.

Making reference to the accompanying drawings and first to Fig. la, an amplifier circuit is set forth wherein two successive stages o amplification are provided by the tubes I and 2. In this circuit arrangement, input signal energy is applied at input terminal 3, while output signal energy is derived from the output terminal d. In a circuit of this type it is apparent that the plate.- to-ground capacity 5 of the first tube I, and the grid-to-ground capacity t of the second tube 2, which are coupled by' the coupling condenser l, act in parallel to shunt the load resistor 8 of the rst tube i. In this circuit, operating plate voltages for the tubes i and 2 are applied at the terminal 9, while suitable grid .bias voltage is sup-- plied at terminal I El, for instance. The cathode elements are suitably grounded, for example, at

In such amplifiers it is customary to avoid the plate--tc-grid capacity by the use of screen grid tubes and separately shielded leads. as is well known.

L", for illustrative purposes, platencground capacity 5 is assumed to be equal to G mmf. and the grid-to-cathcde capacity of the second tube is assumed to be equal to ll mmf. then it is evident that the resultant capacity placed in shunt to the 'output resistor d of the first tube I is 17 mmf., for the tubes alone. This means that with strays considered, the total shunt capacity will be on the order of 20 mmf. or more, and also indicates that for a wide band amplifier the plate resistor will be relatively low in value. Consequently, for any customarily used televisionl type ampliiler tubes (such, for example, as the 6AC7), the gainper stage will be relatively low, and, .for instance, usually of the order of not over 15, where the band width required in a. television ampliiier `circuit approaches 4.5 megacycles.

If now, however, reference is made to Fig. lb, it will be seen that a cathode coupler circuit has been set forth to transfer and transmit the plate voltage swing from the first tube I to the succeeding tube 2. This cathode follower 4couplepgis diagrammatically represented by the tube 'I2,

and, as is known, such` a cathode follower offers .practically no additional grid-to-ground capacgrid and cathode. Consequently, with a circuit of this nature, the only shunt capacity remaining to shunt the output resistor 8 of the tube I is the plate-to-ground capacity 5, plus perhaps some stray capacity for wiring.

The grid capacity 6 of the tube 2 is effectively disconnected from the plate of the driver tube I, as represented by Fig. 1b, and, as indicated, it acts as a load for the cathode-follower tube I2.

No matter what value the cathode resistor II of the tube I2 may happen to be, the cathodefollower stage I2 must be considered as a aen-A erator with the E. M. F. of its input dependent upon tube I and an'equivalent internal resistance of l/a, where g represents the mutual transconductance of the tube I2.

Considering the tube i2, for example, to be a tube of the type known in the art as the 6J5, it can readily be calculated that the gain per stage, with a transmitted frequency band of the order of 6 mc.,may be considerably in excess of 30, which is highly desirable in a. television circuit, and indicates clearly that the available gain for a given band width is more than doubled by the cathode-follower coupling.

In the design of such multi-stage amplifiers, it is always apparent how much gain is required because the input signal strength'is usually known, and, for the purpose of operating television image reproducing apparatus, the actuating signal strength necessary is likewise known. Accordingly, it is easily possible to calculate the necessary gain.

Under such circumstances, it frequently happens that two amplifier stages, cathode-follower coupled,v may be insufiicient to produce the required gain, and yet three amplier stages will produce more gain than is required. Therefore, to take care of such situations, the gain per stage may be reduced by reduction of the plate impedance, but to obtain a more uniform response from the amplifier for wide' frequency bands, it is preferable to introduce cathode degeneration. Such a method makes possible trading a loss in voltage gain for a gain in the band width unifcrmly handled, and is conveniently provided through the aid of a capacitive shunt used in grounded connection with the driving amplier.

if now, reference is made to the modification ci the circuit, as is shown by Fig. 2, it will be seen that a capacitive high frequency shunt 2i is provided in the connection between the cathode of tube I and ground i5. This capacitive shunt 2| is preferably shunted by a cathode resistor 20. Under such circumstances, it may be assumed that tube I is of the type known in the art as the GAC?. y

Several conditions o operation may be considered by making reference to the curves of Fig. 3, wherein the ampliiier gain is plotted against the frequency response. The curve A represents the amplier response condition with degeneration provided by the resistor 2U, but where the capacity C21 of Fig. 2 is equal to zero. From this curve, it can be seen that from some intermediate frequency value F4 the gain begins to fall ofl'gradually. However, from the same curves of Fig. 3 it can be seen, by reference to curve B, that if the capacitor 2| is added and where the time constant of the circuit comprising the capacity 2| andthe resistor 2li is made equal to the time constant of the plate-to-ground capacity 5 and the plate resistor 8 of the ampliiler, the response remains at a substantially uniform val'ucor a somewhat wider lfrequency range (until frequency Fs, for instance) and then falls off more slowly than in the case where the shut capacity 2| is omitted in con- Junction with the degenerating cathode resistor 20.

A third possible operating condition is shown by the curve C where la. greatly peaked output is provided below the frequency value F1, where the amplifier response under condition`A first tends to fall ofi'. However, furtherreference to vcurve C shows that while the amplifier response that provided by the capacity 8 and .the resistor 8. y

It thus is seen that curve C provides substantial peaking but no increased band width.v

Thus, operation of any cathode follower system for a wide band video amplifier is preferable under the conditions of' curve B, provided some peaking effects are introduced in conjunction with this type of operation. Accordingly, reference may now be made to the further circuit modication shown by Fig. 4 wherein such peaking is introduced. With the general type of circuit shown by Fi 2, modification may readily follow (as in Fig. 4) to provide the necessary high frequency peaking. In many eases this high frequency peaking may be provided by virtue of the inductance element 25 connected in series with the load resistor t. 'I'his provides ashunt peaking transmission of th signal over short cable lengths, provided suitable peaking appears also at the output of the cable. This is particularly desirable where different stages of the amplifier are carried in separately shielded units.

Now, referring to Fig. 5,'particular emphasis has been placed upon'a circuit wherein special which is desirable for inter-stage coupling.

Under someconditions, or for conditions where especially heavy loads are to be handled, it frequently becomes desirableto utilize a. series peaking inductance, such as that shown at 28, intermediate vthe cathode output of the cathodefollower I2 and the driven tube 2. Where an inductance of this characteris used, it sometimes becomes desirable to exclude the shunt peaking inductance 2Q, or to short circuit it, al

y Following this type of analysis, it is evident that the capacity 2 :may be made much larger' and within much more reasonable and convenient size with series peaking than is provided by the usual grid-to-sround capacity of the more conventional circuit.

Under some conditions, whereA it is desired to derive an output directly from one cathode-folshielded cable may be connected in p1ace\of the condenser 2 and act as a load for the inductance element or peakingcoil 28 as long as the capacity vvalue is kept within the range VVwhere series resonanceroccurs at the desired peaking frequency. In this way, no appreciable.

losses of picture definitions occur vby reason of When the provisions have been made for low frequency correction, whereby the lowest frequency values may be faithfully transmitted, with the possible inclusion of the D. C. component. In the arrangement of Fig. 5, the circuit elements disclosed are essentially like those of Fig. lb and also Fig. 4, but they provide, in addition, a low-pass filter in the form of the 'resistor 38, which is in series with the plate resistor 8, and the capacity 3l connected between the junction point of resistors 38 and 8, on the one hand, and ground I5 on the other hand. This circuit then acts as a high frequency. trap and, at the same, provides low frequency emphasis. A further adv vantage is provided in that the voltage drop', which takes place through the resistor 80, is also made useful as a plate voltage supply for the cathode follower tube I2, which is connected directly in parallel to the plate supply 9 and the. plate of the' amplier tube I.

Witho the above arrangement, some of the low frequency. waves are predistorted and contain eventoo much low frequency components. Ac-

cordingly, by virtue of the combination provided by-the capacity 85 and the resistor 88, serving as a high-pass filter, this predistortion can becompensated. In this connection, the coupling condenserv88 is usually shunted by a critical'resistor 81, provided that suitable negative bias voltage is supplied to the control electrode of the driven tube 2 from a suitable bias source, such as that conventionally'indicated at 88. Where only the conventional A. C. video amplication is desired, the leakage resistor 8`Il may be omitted.

The arrangement thus described transmits not. only the lowest frequencies of the desired response band, but it also makes provision for transmit@ ting a steady-state condition, or the direct current component through the amplier in a manner which can be readily ascertained from computations. 5

In a slightly further modication of thev arrangement, which is particularly adapted for use where' the condenser 3i is in the nature of an electrolytlc condenser, with perhaps imperfect insulation, the combination may be represented 'lower stage, such as the tube I2, a piece of by the condenser 40 and a series resistor di, as shown by Fig. 6, where the resistor di represents the equivalent losses through thel electrolytic condenser. Accordingly, to provide adequate compensation, a further series combination of condenser H6 and resistor 48 is connected, as in dicated by Fig. 6, in parallel with the condenser and the resistor 87. l

With the cathode coupler circuits hereinabove described, considerable Aopportunity is aorded for the inclusion of the` gaincontrol circuit which has a wide range of operation with complete absence of noise Introduction and,.at the same time, offers no change i'n either frequency response or in the transmission of direct currents. In this connection, the circuit of Fig. 7 is particularly useful, it being understood however, that 0f Fig. 5 with the DIM Stretcher 2l, Il, disclosed by Figs. 2 and 4, also included. In the arrangement of Fig. 7, the cathode follower tube l2 derives its power supply through the resistor I I, but the apparent impedance of the tube I2 is much smaller thanv the actual value ot the resistor Il. Between the cathode element of the tube I2, which cathode is connected to the tube -2 from the point 5d, and a suitable constant voltage source, conventionally represented as the glow-'discharge tube 50, shunted by condenser 5I for supplying voltage by a conductor 51, a variable resistor elenient E52 is connected. The operating conditions of the tube i2 are adjusted so that its cathode voltage is Just equal to the voltage cf the source represented at 5B. The result is that no direct current leaks away through the gain control resistor 52, and it becomes apparent that this potentiometer can be operated without introducbecause there is no D. C. drop across it. Stated in another way, when the voltage across the ing any noise or motor-boating in the system cathode resistor ii is made equal to the voltage straightened out through the use of a logarithmic volume control element 52. Y

Now, making reference to the circuit of Fig. 3, there is provided a niodlilcation which combines theI gain control of Fig. 7 with a picture inverter stage, whereby either positive or negative output energy may be derived. In this connection, signal input energy is supplied, as before, to the input terminal 3, and thence to the control tube i, from which the energy is now fed to an inverter tube 52 which serves to connect the control tube i, in cathode follower fashion, to the driver output tube 2. However, in accordance with the position of the switch element Si relative to contacts t3 and tid will be determined whether the resultant output energy supplied to the driver tube 2 is positive or negative, as will later be eicplained.

In' connection with the arrangement of Fig. 8,

triode, such, for example, as the tube known in the art as the SFB or the 7F?, although other 'tubes may readily be substituted and even separate triodes used forthe rst and second sections thereof. With the system disclosed, the first half or left-hand half of the tube e2 preferably acts as 1:1 voltage amplier provided with a suitable plate resistor of reasonably low impedance. With a switch 6l positioned adjacent the contact point 63, a connection is made to the left half of tube d2 to provide for transmitting a positive picture, while with the switch 6I resting against the contact'p'oint 64, the connection is made to the output of the right-hand half of tube 82 to provide for transmitting` a negative picture. Thus, the right-hand half of the tube I2, which is energized from the output'l of the left-hand half of this tube through the coupling condenser B6, then acts simply asa cathode follower with inverted polarity. v y

Bearing in mind they circuit hereinabove ex,-

, the tube 82 is usually, for convenience, a double plained in Fig. '7, wherein the gain control was particularly described, it will be seen that the same circuit elements are set forth by Fig. 8, and consequently, with the arrangement disclosed, the same "variable resistor or potentiometer unit 52 may serve as a volume or gain control for either polarity of picture, it being noted that the control range preferably should be of the order of about l to 10.

By the arrangement of Fig. 9, a substantially complete form of amplier circuit to include the ampliiier and the introduction of blanking signals, along with the picture or video signals, has been set forth. In connection with such high gain wide band ampliers, it is particularly important that there Ibe a substantially symmetric output; that the voltage supplied be suitably stabilized; and that further stabilization be introduced, particularly for the rst stage of amplication, and what is known as the pre-amplifier" or, in other words, the amplifier stage connected closest' to the picture tube output. With a circuit of this character, stabilization has been indicated as 4being provided by means of suitable glow-discharge lamps or neon lamps, herein conventionally indicated and represented by the lamps lll, I 3l and |42, from which, for purposes of illustration, assumed output voltage of 225 volts and 150 volts `positive relative to ground 15, and Volts negative relative to ground are developed, will be assumed. By making the ampliier output symmetrical, as indicated by Fig. 9, adequate stability of output is maintained with a limited amount of capacity in parallel Yto the several neon lamps or glow-discharge tubes.

To this end, the two output stages Il and 'l2 will be assumed, of which the formez' supplies the output signal energy of positive sign to the output terminal l2? or to the shielded cable 923, as will later be explained, and of which the latter supplies negative output energy to the output 'terminal |26. In connection with the disclosed arrangement, both of the output stages 'li and l2 operate as cathode followers, but the emission from the tube ll is from some assumed minimum stationary current value up to some assumed maximum current value, whereas the emission from the tube 'l2 is from the assumed maximum current value down to the assumed minimum stationary value. The result is that the sum of both )picture modulations just cancels, as can be determined from the common power supply. Under these circumstances, and by this method offbalancing the high power video output, the problem of how to avoid lov.7 frequency feedback and "motor-boating is substantially simplified, and, at the same time, an additional video channel is available to carry the negative modulation.

as is frequently useful in connection with the 25, is extremely important, and this voltage regulation is carried out by means of a stabilizing triode tube 73, connected in such a manner as to provide voltage stabilization.V To this end, the condenser 'F4l is of such size as to transmit even very low uctuatlons of the assumedl 225 volt energy supply connected at the terminal 9 to the grid or control electrode of the tube 13. The

.plate resistor 15 of the tube 'I3 is made adjustable,

. adjustment of the plate resistor of the tube 13 alone.

.As is well known, in connection with the operation of wide band ampliers, it is not usually practical to carry out the amplification of the initial signal energy by direct current amplifiers in all stages. The recourse to A. C. ampliners results inthe loss of the direct current component, due to theuse of capacity connections, but by 'the Ause of a separate diode, or by the use of grid current action 'in one of the tubes oi the latter stages of the amplifier and at one ofthe high amplncation levels, the lost direct current component may be reinserted, as will herein be explained. y A

ln this connection, initial output signal energy, representative of the picture or scene to be televised, results from a scanning action within a scanning camera or picture tube 99 where an optical image of the picture or other scene is suitably focused upon a mosaic element 9| to produce an output signal across the load resistor @2 whenever the vmosaic 9| is scanned by a cathode ray beam in a manner well known in the'art and thereiorevnot herein speciiically disclosed. The tube 90, and the manner in which scanning takes place, may be assumed to be in accordance with the description of such operations by Messrs. Rose and Iams in an article entitled The orthicon, a television pickup tube, which was published in the "RCA Review in October, 1939, or it may be in accordance with the principles and method set forth in an article by the same authors entitled Television pickup tubes using low-velocity electron-beam scanning, which appeared in the Proceedings of I. R. E. for September, 1939. I

When scanning takes place within the picture or camera tube 9B, output signals are developed which are then supplied to the amplifier tube inverter tube 62 is fed by way of conductor |01 to' the inputof the driver stage 80, which precedes the output stages or the endlstages-ll and 12, las was hereinabove set forth. i

D. C. insertion of the lost components, due to thewA. C. connections (as of amplifier 95, for example), may be carried forward by grid current action inone of the higher amplication stages. To this end, the tube 80 is so arranged that it derives zero grid bias through the grid leak resistor 8|. Accordingly, tube 80 operates as .1an audio and consequently draws grid current, but the positive peak values of the grid swing are kept at zero relative to the cathode and never exceed such value. However, in order to avoid excessively heavy current through the tube,.

the screen voltage is considerably reduced, as indicated.

Thus, beginning with the tube 99, by which the D. C. level is set, all of the following tubes are direct current coupled and the low frequency compensation takes place by virtue of the arrangement hereinabove described, particularly in accordance with the circuits ofFigs. 5 and 6, as indicated. To this end, output signals of either positive or negative sign are, with a D. C. component included, derived at terminals |21 and. |26, respectively, in that the output from tube 1l is connected by way of the conductor Hi8 through a compensating network, of the type described in Fig. 6, to the tube 12.

In any amplifier of this character it is desirable that provision be made for injecting or modulating blenking signals into this video sig.- nal output without affecting the D. C. insertion. In the arrangement of Fig. 9, these blanking signals are injected into the stage 1|, with a cathode V feeder resistor |50, which follows the driver stage.

99, commonly known in the art as the preamplier, in that it is usually closely associated with the camera tube. VThe signals resulting from scanning are then amplied and supplied incathode follower fashion, by, way of the peaking inductance $6, to the transmitter cable 94. The peaking, by virtue of inductance 9B, occurs inthe manner described in connection with Fig. 4,

with the there described capacity element 21 being represented by the vcapacity of the cable 94. The output from the cable 94 is now supplied to the first amplifier l96 whose output is supplied through a ydirect connection to the cathode follower tube ss. The output of: the cathode follower tube 96 is'then fed to the next amplifier stage 91, through a. peaking inductance 26, as was'explained by Fig. 4.. 'In this connection, it may also be seen that the-low frequency compensation is provided by the resistor 30 and ghe capacity 3| connected in the output of tube The output energy for the second amplier tube 91 is supplied to the inverter or signal reversing tube 62 (as described in connection with Fig. i!) to provide, at the switch 6|, either positive or negative picture outputs. "From the switch elementi 6|, signal energy output from the Since the output stages in the circuit arrangement disclosed operate as cathode followers, the blanking signals are injected upon the yscreen electrodes. To this end, the blanking. signals, which have been developed in any` suitable sync signal generator (not shown), are applied at the terminal input 83 and fed through the 'coupling condenser 3B to the control electrode or grid of the right-hand half of the twin triode 82. In this connection, grid current, as indicated, provides adequate negative bias. The ,/blanking signals, yas they are amplified across the load resistor 85 connected to the vright-hand section of the tube 82, are then coupled through the coupling condenser 96 to the control electrode of the left-hand half of the tube 82, which electrode is preferably connected to the voltage source at the terminal 51- by way of the resistor 81. By virtue of the conductor 88, the cathode element lof the left-hand half of the tube 82 connects directlyrto the screen grid of theeoutput stage 1|- and, similarly, a, connection is made also tothe screen electrode of the second output tubeg12. In this connection, 'the arrangement disclosed develops adequate power ato take 1|, and to keep the screen slightly above the in,

' dicated 150 volt potential at terminal 51 during the line sweep while it modulates the screen grid to a vpoint below the cut-olf value during the` blanking period. The connection shown provides extreme freedom from hum which may exist across the high voltage line at terminal 9 and also oiers low. impedance for the screen circuitv and abilityvfor high-speed keying.

With this circuit arrangement disclosed by the heater voltage line |02 which 'supplies the 5 heater voltage to the various tubes, of which the heater |30 has been conventionally represented. The bleeder resistor supplies compensating voltage of variable amplitude, which is further divided by the bleeder resistor combination |03 l0 and |00, with the result that a compensating 60 cycle voltage (a 60 cycle source assumed) is impressed upon the screen electrode ofthe rst amplifier stage 95 where it is made to eliminate the effects caused by the magnetic field due to i5 the heater current. To eliminate further hum from the picture itself, a 'phase shifting circuit is also preferably included between the screen electrode 95 and the bleeder combination |03 and |04. Such a phase shifting network comprises condenser |09 and resistor H0, of which either or both elements may be made variable. The result is that it becomes unnecessary to provide\ for D. C. operation even for the heater of the first stages. However, in order to avoid any effects of high frequency feedback, the heater conductors |02 are blocked to ground by relatively large condensers connected at three points in the system, which may be illustrated, by way of example, by the condensers H5 and ||0 con- 80 nected in the pre-amplier stage, the condensers ||1 and ||8 connected in the main amplifier stage, and by the condensers ||0 and |20 which are indicated as in the power stage where the heater element |3| conventionally represents the heater cathodes of this stage.

Suitable power for the operation is derived from the alternating current source connected at |00 andl feeding energy into the transformer primary |34 from which the heater current is derived by way of secondary winding |02. The high voltage is derived by way of the secondary winding l connected to the rectifier |31, in known manner,

with the illter condensers |38 and |30 functioning s without departing from 'the spirit or scope of what s is set forth.

Having described the invention, what is claimed is:

1. An ampliner circuit including' a signal input amplier -tube and a signal Aoutput amplifier tube, an electronic cathode follower coupling tube having at least a cathode, a control electrode and an anode, means to supply the output energy from the signal input ampliiier to the control electrode of the said coupling tube, a cathode load resistor connected with said coupling tube, means to supply energy to the input of said signal output ampliiier in accordance with the energy output of the said coupling tube as it appears across the said cathode resistor, a shunt connected inductive peaking element connected in the output cir. 05

cuit of the signal input amplifier tube and a series connected inductive peaking element connected intermediate the cathode output o! said cou lin tubeandth inputoithesignalout t\ p g e pu 70 cathode output of one or the other of said thermtube.

k2. An amplifier circuit including a signal input amplifier tube and a signal output ampliiler. tube, an electronic coupling tube having at least a cathode. a control electrode and an anode, meanstosupplytheoutputenergyfromthesignal input amplier to the control 'electrode to the said coupling tube, a cathode load resistor connected with said coupling tube, means to supply energy to the input circuit of said signal output amplier in accordance with the energy output of the said coupling tube as it appears across the said cathode resistor, a voltage source adapted to. develop output energy of substantially the same magnitude as the voltage drop produced through the cathode load resistor of the coupling tube,a variable resistor connecting the said voltage source to said cathode resistor at a point adjacent its connection with the cathode of the coupling tube so that signal output may be noiselessly controlled by adjustment of said resistor, and means for applying the output energy of said 'coupling tube to the input of the signal output tube.

s. The circuit claimed in claim 2 comprising,

in addition, resistor4 means connected with the" input circuit of the said signal output tube for biasing the said tube, whereby predetermined signal output voltages may be maintained.

4. An ampliiier circuit comprising at least a rst amplier tube and a, second amplier tube connected in cascade, means for supplying video signall energy to the input circuit of the iirst of said tubes, a cathode-follower coupled thermionic tube connected to receive in its input circuit the signal output from the iirst of said amplifier tubes and to deliver its cathode signal output to the input circuit of the second ofI the said ampliers, resistor and condenser combinations connected ln the output circuit of thefirst of said ampliiler tubes and in the input circuit of the second of said amplier tubes and the output circuit of the coupling tube to compensate for low frequency distortion in signal transfer from the first to the second of the amplifier tubes. 5. The circuit claimed in claim 4 comprising,

in addition, a parallel resistor and condenser combination connected in the cathode circuit of the first of the ampliier tubes to provide controlled degeneration and to increase the signal band width amplied,

' 6. The amplifier claimed in claim 4 comprising, in addition, a resistor element connected in series with each of the cathode and anode elements of the first amplifier tubes, and a .condenser con- 0 nected to one` terminal ot each of said resistors and to a point of iixedpotential, the condenser and resistor combination connected in the cathode circuit of said tube being adapted to be ad- Justed so that the time constant thereof is substantially equal to the time constant of the resistor and condenser combination connected in o second amplifier tube, means tosupply signal energy to the input circuit of the first ampliiier tube, a plurality of thermionic coupling tubes connected intermediate the first and the second ampliiier tubes, a cathode output circuit for each of the said thermionic coupling tubes, means to supply the output energy from the mst of said coupling tubes to the input of the second coupling tube, switching means to connect the input ofthe second of said ampliner tubes to the ionic coupling tubes, whereby signals of like and dinerent polarity to the, signal output 'energy of the nrst amplifier tube are transferred to the input of the se'cond amplifier tube in selected negal tive and positive polarity.

8. A video lampller circuit for 'television comprising at least a iirst amplier tube and a. second ampliiier tube, means for supplying video signal energy to the input circuit of the first of said tubes,

a cathode-follower connected thermionic coupling distortion in signal transfer from the first to the second of the ampliiler tubes, and inductive means for peaking the higher frequency energy supplied to the second of the amplifier tubes.

9. The circuit claimed inclaim 8 comprising, in

addition, a parallel resistor and condenser comleinatlcn connected in the cathode circuit of the Erst of the amplier tubes to provide controlled degeneration and to increase the signal band width amplified.

1o. The empnoer claimed in claim c ecmprie- "25 ing, in addition, a resistor element connected in series with each of the cathode and anode elements of the iirst amplifier tubes, and a condenser connected toone terminal of each of said resistors and to a point of fixed potential, the condenser and resistor combination connected in the cathode circuit of said tube being adapted to be adjusted so that the time constant thereof is substantially equal to the time constant of the resistor and condenser combination connected in the anode circuit, whereby partial degenerationand increase in signal band ampliiica'tion result.

ll. A television signal amplifier for amplifying uniformly a relatively wide band" of frequencies comprising a iirst and a second amplier tube, 4

means to supply signal energy to the input circuit of the first of said amplifier tubes, a triode coupling tube in cathode-follower connection for supplying the signal output of the first amplifier tube to the signal input circuit of the second amplifier tube as a. cathode load on the coupling tube, whereby the capacity load of the second amplifier tube on the first ampliner tube is substantially eliminated, a resistance and condenser combina.-

tion connected in the output circuit of the rst ampliiier tube for enhancing low frequency gain, an inductive peaking element serially connected in.` the input circuit of the second amplifier tube,

'and a capacity connected to load said inductiveelement so that said elements provide for peaking in the higher frequency .range of signals supplied to the second amplifier tube, a degenerating cathode resistor connected in the cathode circuit of the first amplifier tube and a condenser connected in parallel therewith whereby degeneration and band width stretching are introduced. y

12. A television signal amplifier for amplifying Y uniformly .a relatively wide band of frequencies comprising a first and a second amplifier tube means to supply signal energy to the input circuit of the iirs't ofsaid amplier tubes, atriode coupling tube in cathode-follower connection for supplying the signal output of the iirst amplier tube to the signal input circuit of the second amplifier tube vas a. cathode load on the coupling tube, whereby the capacity load of the second amplier tube on the first amplier tube is substantially eliminated, a resistance and condenser combina.-

c ,ssaacs v e 7 amplifier tube for enhancing low frequency gain,

"an inductive peaking element serially connected in the input circuit of the secondl amplifier tube, a capacity connected to load said inductive element so that said elements provide for peaking in the higher'frequency range of signals supplied to the second amplifier tube. adegenerating cathode resistor connected in the cathode circuit of the nrst amplifier tube and a condenser connected 0 inparallel therewith whereby degeneration and band width stretching are introduced, a. third amplifier tube, a signal inverter tube connected in cathode-follower manner to supply the output energy of the second amplifier tubeto the third ampliiier'tube, a switching means toconnect the said cathode-follower signal inverter to' the third amplifier tube to supply said-third amplifier tube with signal input energy of positive or negative polarity, a terminal for connecting a source of alternating current to supply heating current to all of said tubes, and potentiometric means and capacity means to compensate forhum conditions introduced by said alternatingheater current.

13. A television amplifier circuit including a ilrst and a second amplier tube each having included therein at least a cathode, an anode, andV two grid electrodes of which-'one is a signal controlled element and the other of which is a screening element, an anode impedance for the rst of said tubes which comprises a series combination of a resistance element, an inductive element and 5 frequency emphasis is added to signal input energy applied to the signal controlled grid, a coupling triode tube in cathode-follower connection between the first and the second of said amplifier tubes, said coupling triode having a cathode, an anode and a control electrode and having the control electrode and anode circuit directly connected in parallel with the said plate impedance of the rst of said amplier tubes, a resistor connected to the cathode of the coupling triode to control the current iiow in the said tube, a capacity and resistance combination forming a high-pass' filter combination connected between the cathode of the coupling tube and the signal control grid of the second amplier tube, said capacity and resistance combination having a. time constant equal to that of the said low-pass iilter circuit so as to provide substantially uniform gain for the low frequencies in said ampliers,

and an output circuit for the second of said amplifiers. p

14. A television amplifiervcircuit comprising a plurality of-groups of amplifier units each includ-f ing an amplifier tube having included therein at least a, cathode, an anode and two grid electrodes of which one is a signal controlled element and the other of which is a screening element, an anode impedance for the said tube which comprises a. series combination of a. resistance element, an inductive element and a capacitiveresistance combination constituting aflow-pass filter through which a direct current voltage drop occurs and in lwhich low frequency emphasis is added to signal input energy applied to the signal controlled grid, a, coupling triode tube in cathodefollower connection, said coupling triode having a cathode. an anode and a control electrode and having the control electrode and anode circuit directly connected in persue1 with the said plate tion connected in the output circuit er the mst 7s impedance of the rst of sold' emplier tubes. s

Y resistor connected to the cathode of the coupling triode to control the current flow in the said triode, a capacity and resistance lcombination forming a, high-pass filter combination connected to the cathode of the coupling triode, said capacity and resistance high-pass lter combination having a. time constant equal to that of the low-pass 5 amplier tube of the following group of tubes.

KURT SCHVLESIIINGER. 

